Graphene is generally a hexagonal arrangement of carbon atoms forming a one-atom thick planar sheet. The successful isolation of single- and few-layer graphene by the mechanical cleaving of highly ordered pyrolytic graphite (HOPG) has led to a significant increase in studies in numerous research areas. Of the many interesting properties of graphene (such as superior electron and hole mobility (up to 200,000 cm2 V−1s−1), high current carrying capability (up to 3×108 A cm−2)), its uniformly high transparency in the visible and near infrared region, with good electrical conductivity and mechanical robustness, place graphene as a promising candidate for an alternative to indium tin oxide (ITO) as a transparent conducting electrode (TCE).
Several criteria, such as electrical conductivity, optical transmittance, and work function (WF), need to be optimized for the integration of graphene sheets as TCEs in organic photovoltaics (OPV). Generally, graphene has high transmittance with moderate conductivity. An important factor, however, is the energy level alignment between the work function of graphene and the highest occupied molecular orbital (HOMO) of the electron donor material. A WF value of 4.27 eV (electron volts) for a monolayer of graphene synthesized from LPCVD has been observed, which is lower than for ITO (˜4.5 eV and can be increased up to ˜5.0 eV after oxygen (O2) plasma treatment). This low value for graphene is not a good match for the electron donor material (such as tetraphenyldibenzoperiflanthene (DBP), HOMO=5.5 eV), copper phthalocyanine (CuPc), (HOMO=5.2 eV) or poly(3-hexylthiophene) (P3HT), (HOMO=5.2 eV), which can induce a large energy barrier at the interface between the graphene and the organic layer.
For ITO anodes, a thin layer of conducting polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), may be inserted before the deposition of the electron donor material in order to favor an ohmic-contact at the junction. The PEDOT:PSS hole transporting layer (HTL) with a WF of 5.2 eV facilitates the injection/extraction of holes and/or helps planarize the rough surface of the ITO, which often becomes a possible source of local shorting through the ultra-thin active layers, thus improving the overall device performance. Therefore smooth and complete coverage of the PEDOT:PSS layer on the underlying electrode surface plays a crucial role in the general OPV device performance. Application of PEDOT:PSS onto the graphene surface has been challenging due to the fact that graphene surface is hydrophobic but PEDOT:PSS is in an aqueous solution. The sputtered ITO surface is also hydrophobic but it is almost always pretreated with O2 plasma, which renders the hydrophobic surface into an hydrophilic one by introducing hydroxyl (OH) and carbonyl (C═O) groups that enables conformal coverage of PEDOT:PSS. Active oxygen species from the plasma disrupt the aromatic rings of the graphene and greatly reduces the conductivity. In the case of single layer graphene electrodes, a graphene film can completely lose the conductivity after such plasma treatments.
Recently, the wettability of PEDOT:PSS on the graphene surface was improved by doping it with gold (III) chloride (AuCl3). However, the doping process introduces large Au particles (up to 100 nm in diameter), which can create shorting pathways through the device. This method is less favorable also due to the high cost of AuCl3 dopant. Molybdenum oxide (MoO3) HTL with acid-doped graphene electrodes, which is a common HTL material used with ITO electrodes has also been used. However, the device performance was not as efficient as the ITO control device with a MoO3 layer alone and still required the use of PEDOT:PSS on top of the MoO3 interfacial layer, which allowed better wetting of PEDOT:PSS on the MoO3-coated graphene.
Accordingly, improved devices, systems, and methods are needed.